Some mechanistic understanding of the impulse strength of nanocomposites
Yujie Hu, Robert C. Smith, J. Keith Nelson and Linda S. Schadler
Rensselaer Polytechnic Institute, Troy, NY12180, USA
Abstract: Improvements in the dielectric properties of
composite dielectrics have been previously documented
when the filler material used is reduced to nanometric
dimensions. While the reasons for this have been traced
to the physics and chemistry taking place at the
interface, and dramatic changes in the magnitude and
dynamics of the internal charge are also known to occur,
a clear picture of the exact mechanisms taking place has
not emerged. This contribution seeks to compare the
direct voltage breakdown of composites formed from
biphenyl epoxy resin and titanium dioxide in both
nanometric and conventional micron-scale forms with
that obtained under impulse voltage conditions. The
same materials are subjected to an internal charge
analysis using the pulsed electroacoustic technique
which shows that, in the case of the nanomaterials, a
marked homocharge is formed in front of the cathode
which would suggest that the dramatic improvements in
voltage endurance seen for these materials may be due
to the shielding effect of this negative charge. The
finding also suggests that the negative charge is formed
as the result of scattering occurring in the nanodielectric
which is not present to the same extent in the
conventional counterpart.
Background
Extensive interest in the formulation of nanodielectrics
is driven by the promise of both enhanced dielectric
properties and by the possiblility of simultaneously
tailoring the mechanical and/or thermal properties [1] of
dielectric materials where those properties are pivotal in
specific application areas (e.g. insulation for cryogenic
systems). The electrical property of greatest interest is
electric strength, and enhancements for nanocomposites
have been documented [2,3]. In contrast, conventional
composites formed using particulates of micron scale
tend to substantially reduce the electric strength of the
resulting composite.
The emerging explanation for improved electric
strength in nanocomposites relies on the dominance of
the interface region, or interaction zone, surrounding the
included particles. When the particles are less than
100nm in diameter, the interface regions are so large
that they start to interact even at quite modest particle
loadings (typically about 1-10% by weight). In this
situation the composite properties depend on the nature
of the interface layers and are not necessarily dominated
by the properties of the original constituents. In that
sense, such nanodielectrics are entirely new materials,
and the properties can be adjusted, in principle, by
making changes to these interface regions through the
chemistry associated with the interface. This may be
done, for example, by changing the functional groups on
the particle surface [2].
One example of the impact of the interface region is
that there are striking changes in the internal space
charge profiles associated with nanocomposites [4]. In
particlular, the Maxwell-Wagner interfacial polarization
normally associated with composite materials is
mitigated and the levels of internal charge substantially
modified. This contribution seeks to examine the
relationship between the internal charge dynamics and
the electric strength of this new class of materials when
subjected to time-varying stresses.
Nanodielectric formulation and Testing
The composite system reported here is an epoxy-TiO2
combination utilizing 23 nm diameter nanoparticles (or
1.5 µm microparticles for comparison) which were
vacuum treated at 195°C for 12 hr, and compounded
with the Diglycidyl Ether-Bisphenol A resin (Vantico
CY1300) using a SpeedMixer Model DAC 150 FV-K
from Hauschild utilizing a dual asymmetric centrifuge
(DAC) mixing method described previously [5]. A cross
linking protocol using an amine-based cross-linking
agent (Vantico HY956) was undertaken at 25°C for 48
hours followed by 60C for 3 hours to create recessed
samples
for
electric
strength
measurements.
Microscopic checks were made to assess dispersion and
to reject any samples with obvious flaws. It has been
found that the DAC method has substantially improved
the dispersion which reflects in improvements in
breakdown characteristics when compared with those
obtained with earlier methods [4].
Impulse electric strength for the base resin,
nanocomposite and microcomposite was measured
using recessed specimens with thicknesses ranging from
50 to 500 μm with metallized electrodes, using a
standard 1/50 μs waveform. Comparative short-term DC
and AC tests were also conducted with a ramp rate of
500V/s.
Experimental results
Table 1 which shows no measurable improvement. All
the samples have a large impulse ratio (IR) also
Electric strength
Micro Base Nano
Strength (MV/cm)
In order to provide a baseline measurement for the
impulse electric strength, Fig. 1 depicts the DC shortterm breakdown strengths of the three materials shown
6
5
4
3
2
1
0
0
0.005
0.01
0.015
Thickness (cm)
Figure 3: Impulse breakdown strength of composite formulations
documented in Table 1.
Internal charge accumulation
In order to better understand the way in which the
Nano
Charge (C/m 3)
as Weibull plots. The expected reduction in electric
strength for the micron filled epoxy is clear, but the
nanocomposite shows a 18% improvement in strength.
This improvement over the base resin is in agreement
with findings for other nanodielectrcis [2,6] and results
from the improved dispersion now obtained with the
DAC compounding.
A similar result for the alternating voltage case is
15
10
5
0
-5
-10
-15
-2.E-04
0.E+00
2.E-04
4.E-04
6.E-04
8.E-04
Thickness (m)
Charge (C/m 3)
Micro
given in Fig. 2. Although the nanocomposite still shows
the highest breakdown strength overall, enhancement
in comparison with the base resin was somewhat
smaller than it was in the DC breakdown test.
Fig. 3 shows the corresponding impulse strength as
a function of the gap spacing from which the three
materials appear almost indistinguishable. The wellknown volume effect is clearly visible (as is it for the
other voltage applications), and the nanomaterial
appears to have little advantage over the base resin
when gauged on the basis of impulse breakdown
characteristics. This is evident by examining the
associated Weibull size parameter, α, shown later in
25
15
5
-5
-15
-25
-1.E-04
1.E-04
3.E-04
5.E-04
Thickness (m)
Figure 4: Internal charge profiles for nano (top) and micro
(bottom) composites. Cathode on the left. [6]
particulate size affects the accumulation and dissipation
of internal charge, the Pulsed Electroaccoustic Analysis
technique (PEA) was employed to examine charge
profiles on laminar 0.5 mm samples. Typical results are
shown in Fig. 4 for the case of the micro- and nanocomposite. When high-voltage (~ 10 kV) is applied to
the dielectric it polarizes and image charges appear on
the electrodes. In addition charge may be injected
which will contribute to the image charge. These
trapped space charges are more easily seen after the
voltage has been removed (shown arrowed). It can be
seen from Fig. 4 that, in the case of the nanodielectric,
the predominant charge is a homocharge (negative)
which appears in front of the cathode (on the left in Fig.
4) – the associated positive charge is its image on the
cathode. This is in striking contrast to the case of the
conventional (micro) material in which the opposite
situation occurs and the charge in front of the cathode is
positive. The effect of this charge profile difference on
the internal field may be seen in Fig. 5 where the
reduction of the filler size reduces the electric field in
front of the cathode from about 23 MVm-1 to 18.5
MVm-1 – for a situation for which the average gradient
Electric Field (MV/m)
0.0
-5.0
(a)
(b)
Table 1 summarizes the results of the breakdown study
in terms of the Weibull parameters of the formulations,
based on the Bernard estimation method. The AC values
are given as peak quantities to provide a comparison.
The results clearly show that the nanoscale fillers lead
to higher breakdown strength, and that the micron scale
fillers erode the breakdown strength.
Equally
important, the changes in breakdown strength depend on
the type of voltage application.
The charge profiles of Fig. 4 strongly suggest that
the heterocharge formed in front of the cathode for the
DC
-10.0
AC
(pk)
-15.0
-20.0
1/50
Imp
-25.0
-30.0
-0.2
0.1
0.4
Figure 5 Computed E-field profile in (a) micro- and (b) nanocomposites
is exactly 20 MVm-1.
Of equal significance in the context of the impulse
strength are the dynamics of the charge accumulation,
shown in Fig. 6. It is observed that the maximum
internal field changes only slowly from its nominal
18
17
Micro
16
Nano
15
14
13
50
100
α
β
α
β
α
β
IR
Base
3.32
10.56
2.33
6.54
4.37
5.34
1.88
Micro
3.00
8.47
1.95
6.77
4.29
8.52
2.20
Nano
3.91
10.39
2.55
7.52
4.46
9.20
1.75
0.6
Position in sample (mm)
Electric Field (kV/mm)
Appraisal and Discussion
Table 1: Summary of Weibull scale (α) and shape (β) parameters for
epoxy-TiO2 formulations
5.0
0
Furthmore, there is a difference in the charge decay time
constant (2210 s for nano and 6300 s for micro)
150
200
Time (s)
Figure 6: The temporal development of the maximum internal
field for 10 % TiO2/epoxy nano- and micro-composites at an
average stress of 15 kVmm-1
(V/d) value (enhancement in the case of the
microcomposite and reduction for the nanomaterial).
microcomposite would enhance the local field. This
enhancement is very likely to result in the lower DC
breakdown voltages recorded. Indeed, the enhancement
of the field over the nominal Laplacian value associated
with the charge profile shown in Fig. 4 is calculated to
be 15%. Although this is modest, the stress for this
experiment is only about 1/10 of that at breakdown
where the effect can be expected to be very significant.
By the same token, the formation of negative charge in
the case of the nanomaterial screens the electrode with
the opposite desirable consequence of supporting a
greater voltage before breakdown occurs. This is very
consistent
with
previous
electroluminescence
measurements [4] in Epoxy-TiO2 materials, where the
onset voltage for luminescence was increased by over
100% when the oxide particles were reduced in size,
suggesting a shielding of the electrode.
However, it is also clear from Fig 4 that the
accumulation of this shielding space charge is a process
that requires several minutes. As a consequence, it is not
surprising that the improvements in electric strength
attendant on small particulates are eroded when power
frequency alternating voltages are used, and the
materials are also virtually indistiguishable under surge
voltage conditions. However, this does not detract from
the promise of nanocomposites since the principal
advantage is the enhanced voltage endurance, which can
be increased by up to two orders of magnitude under
AC conditions [2] for divergent field conditions. In
addition, the Weibull shape parameter, β, in Table 1
shows an increase for the inclusion of nanoparticles.
This has also been observed by others [6], and would
indicate that the breakdown phenomena in
nanodielectrics is more consistent, and thus dependable,
than the base resin from which they are formed.
What is not as clear, however, is the reason for the
reversal in the sign of the space charge when the
particles are reduced to nanometric dimensions. Prior
data, however, will help shed light on a possible
mechanism.
First, thermally-stimulated current
measurements on these composites [4] have
demonstrated a much stronger space charge peak for the
microcomposites compared to the nanocomposite.
Furthermore, Table 2 provides estimates of the trap
depths extracted from the thermally-stimulated current
Table 2: Trap depth estimates from thermally-stimulated
current measurements for epoxy-TiO2 formulations
Base
Resin
~1.39eV
Microcomposite
Nanocomposite
~1.44eV
~1.96eV
data by the initial rise method and shows that the
nanomaterial is characterized by significantly deeper
traps. Finally, reference [4] shows that luminescence
and electroluminescence spectra shift to lower
frequencies as the size of the oxide particles is reduced.
This is a clear indication that the environment of the
light emitting center is changed and points to the impact
the large interfacial region in nanodielectrics has on the
charge behavior.
Based on the results in this paper, and those
mentioned above, we offer, despite some reservations, a
working hypothesis. If is assumed that electrons injected
from the cathode can create impact ionization, then a
positive space charge would be expected in front of the
cathode. In contrast, charge trapping in the large
interfacial area associated with nanocomposites could
be expected to generate the shielding homocharge seen.
Lewis [8], in a series of papers, has suggested that these
interfacial regions are akin to the well-known
electrochemical double-layer in liquid media. Using this
as a model, it seems likely that electron scattering is
predominant in such regions [3] and the thermallystimulated current measurements indicate that these
charges are deeply trapped (see Table 2). The scattering
reduces the impact energy preventing ionization, and the
deep traps reduce charge mobility. As a result,
homocharge develops. This behavior has previously
been documented by Hibma et al. [9], and the local
conductivity associated with such overlapping layers
would explain the differences in the time constants
experienced in both the internal charge behavior and the
electroluminescence [4].
Finally, it is perhaps appropriate to observe that,
although some of the features of the various
nanodielectric systems are common (particularly the
attractive augmentation of voltage endurance), the
detailed behavior is by no means universal. For
example, a reduction in the onset of charge injection has
been observed in ethylene vinylacetate/silicate
nanocomposites [6], yet shown to increase, and be
highly dependent on particle functionalization, in
polyolefin nanodielectrics [10]. If, indeed, these
materials are entirely dominated by the overlapping
interfacial zones which can be as much a 10nm in extent
[8], these anomalies should not be surprising since the
interface chemistry will differ.
References
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Author address: J. Keith Nelson, Rensselaer
Polytechnic Inst., Troy, NY 12180-3590, USA Email:
k.nelson@ieee.org